Presently, a variety of uplink channels for use in a communication system have been discussed, for example, a random access channel (RACH) for allowing a user equipment (UE) to access a Node-B, and an uplink shared channel (e.g., HS-DPCCH) for transmitting a channel quality indicator (CQI) and ACK/NACK information.
The RACH channel allows the user equipment (UE) to establish downlink synchronization with the Node-B, and can be found by information of the Node-B. Location or other information of a corresponding channel can be recognized on the basis of the Node-B information. The RACH channel is the only one channel to which the user equipment (UE) unsynchronized with the Node-B gains access.
If the user equipment (UE) transmits a signal to a corresponding Node-B over the RACH channel, the Node-B informs the user equipment (UE) of not only correction information of an uplink signal timing point at which the user equipment (UE) is synchronized with the Node-B, but also a variety of information capable of enabling a corresponding UE to access the Node-B. If the user equipment (UE) is connected to the Node-B over the RACH channel, it may communicate with the Node-B over other uplink channels.
FIGS. 1 and 2 are conceptual diagrams illustrating a variety of processes encountered when the user equipment (UE) establishes an uplink communication with the Node-B.
If the user equipment (UE) accesses the RACH channel, it can acquire both uplink and downlink synchronizations associated with the Node-B, so that it can access a corresponding Node-B.
FIG. 1 shows a specific state in which the user equipment (UE) is powered on and is firstly connected to the Node-B. FIG. 2 shows another state, in which the user equipment (UE) is unsynchronized with the Node-B after establishing synchronization with the Node-B, or it must request uplink resources from the Node-B after establishing synchronization with the Node-B (i.e., it requests resources for uplink transmission data).
As shown in step (1) of FIGS. 1 and 2, the user equipment (UE) transmits an access preamble to the Node-B. If required, the user equipment (UE) may further transmit a message to the Node-B. Therefore, the Node-B recognizes why the corresponding user equipment (UE) accesses the RACH channel, so that it can conduct a necessary process corresponding to the recognized reason.
In the case of the initial access shown in FIG. 1, the Node-B assigns timing information and uplink data resources to a corresponding user equipment (UE), so that the user equipment (UE) can transmit uplink data as shown in step (4) of FIG. 1.
In the meantime, the exemplary case of FIG. 2 indicates that the user equipment (UE) accesses the RACH channel due to a scheduling request.
Referring to FIG. 2, the Node-B assigns timing information and resources for the scheduling request (SR) at step (2). The Node-B receives the scheduling request (SR) from the user equipment (UE) at step (3), and assigns uplink data resources to the user equipment (UE) at step (4), so that the user equipment (UE) can transmit uplink data to the Node-B at step (5).
If the user equipment (UE) accesses the RACH channel using the case of FIG. 2 instead of the initial access of FIG. 1, it is determined whether the signal transmitted to the RACH channel has been synchronized with the Node-B, so that the user equipment (UE) may transmit different signals according to the determined result.
FIG. 3 is a configuration diagram illustrating a RACH signal structure used for a synchronization access and a non-synchronization access.
In the case of the synchronization access, the user equipment (UE) accesses the RACH channel on the condition that it has been synchronized with the Node-B and have continuously maintained the synchronization with the Node-B.
In this case, it should be noted that the synchronization between the user equipment (UE) and the Node-B can be maintained by either a downlink signal or control information (e.g., a CQ pilot) delivered to an uplink. The Node-B can easily recognize the signal contained in the RACH channel. And, because the synchronization between the user equipment (UE) and the Node-B has been maintained, the user equipment (UE) may use a longer sequence shown in an upper part of FIG. 3, or may transmit additional data to the Node-B.
In the case of the non-synchronization access, if a non-synchronization state between the user equipment (UE) and the Node-B is provided while the user equipment (UE) accesses the Node-B, a guard time shown in a lower part of FIG. 3 must be established while the user equipment (UE) accesses the RACH channel. The guard time is established and determined in consideration of a maximum round-trip delay which can be owned by the user equipment (UE) which desires to receive any service from the Node-B.
Besides the above-mentioned synchronization and non-synchronization accesses, the RACH channel must satisfy different requirements according to locations of the UE within a cell (hereinafter referred to as UE's in-cell locations).
FIG. 4 is a conceptual diagram illustrating different requirements according to UE's location within a cell.
Referring to FIG. 4, an edge area of a cell supported by a Node-B is determined to be “R3”, a UE existing in the R3 area is determined to be “UE3”, a specific area existing in an intermediate part of a cell is determined to be “R2”, a UE existing in the R2 area is determined to be “UE2”, a specific area close to the Node-B is determined to be “R1”, and a UE existing in the R1 area is determined to be “UE1”. Detailed descriptions of the above-mentioned cases are shown in FIG. 4.
Referring to FIG. 4, a path loss of the UE1 is denoted by Lp1, a path loss of the UE2 is denoted by Lp2, and a path loss of the UE3 is denoted by Lp3. A round-trip delay (RTD) of the UE1 is denoted by 2td1, a round-trip delay (RTD) of the UE2 is denoted by 2td2, and a round-trip delay (RTD) of the UE3 is denoted by 2td3.
In this case, 2td1 indicates that the RTD is double the delay value of td1 consumed for one-way transmission, 2td2 indicates that the RTD is double the delay value of td2 consumed for one-way transmission, and 2td3 indicates that the RTD is double the delay value of td3 consumed for one-way transmission.
Generally, the longer the distance, the higher the path loss, resulting in Lp1<Lp2<Lp3 and 2td1<2td2<2td3.
Therefore, the lengths Gd1, Gd2, and Gd3 of individual guard intervals required according to in-cell locations of UE1, UE2, and UE3 are denoted by Gd1<Gd2<Gd3. The expansion coefficients Sp1, Sp2, and Sp3 of sequences to be applied to the channel are denoted by Sp1<Sp2<Sp3.
In other words, compared with the UE1, the UE3 must access the RACH channel with a sequence having both a longer RACH and a higher expansion coefficient in order to acquire the same performance as that of the UE1 which accesses the RACH channel with both a shorter RACH and a lower expansion coefficient.
The UE1 uses the RACH channel assigned by the Node-B. However, if a cell radius is very long, the RACH size is designed to be appropriate for a predetermined condition for supporting an edge UE (e.g., UE3) of the cell.
Therefore, if any UE such as the UE1 is located close to the Node-B, the UE need not use the long RACH. The case of FIG. 4 indicates that a time length of the UE1's RACH is longer than that of the UE3's RACH and a bandwidth of the UE1's RACH is wider than that of the UE3's RACH.
The above-mentioned method which satisfies different conditions required for the RACH channel according to the UE's location within a cell to effectively perform the RACH communication, and implements effective communication by defining the RACH length/width and sequence in different ways to implement effective communication, has been disclosed in Korean Patent Application No. 2006-74764 filed by the same applicant as the present invention, entitled “METHOD FOR TRANSMITTING/RECEIVING SIGNAL IN COMMUNICATION SYSTEM”, and Korean Patent Application No. 2006-92835 filed by the same applicant, entitled “RANDOM ACCESS CHANNEL FOR SEGMENTED ACCESS, SEQUENCE, AND METHOD AND APPARATUS FOR TRANSMITTING SIGNAL USING THE SAME”.
And, another method, which requires RACH access reasons different in UE's locations within a cell, and differently allocates the RACH sequence used for each area within the cell according to the relationship between the RACH sequence and another sequence used for a neighboring cell, has been disclosed in Korean Patent Application No. 2006-87290 filed by the same applicant as the present invention, entitled “SEQUENCE SET FOR RANDOM ACCESS CHANNEL, AND METHOD FOR DEFINING THE SEQUENCE SET, AND METHOD AND APPARATUS FOR TRANSMITTING SIGNAL USING THE SEQUENCE SET”, and Korean Patent Application No. 2006-92836 filed by the same applicant, entitled “SEQUENCE ALLOCATION METHOD, AND METHOD AND APPARATUS FOR TRANSMITTING SIGNAL USING THE ALLOCATED SEQUENCE”.
The above-mentioned methods can effectively use resources according to the UE's location within a cell, and can access the RACH channel. If different sequences are allocated to individual areas, the above-mentioned methods can reduce the possibility of generating a RACH collision caused by the same sequence, and can increase the number of random access opportunities of each UE.
In order to allow the user equipment (UE) to access the RACH, the user equipment (UE) must select/transmit predetermined signals. The best sequence from among the predetermined signals is a Constant Amplitude Zero Auto-Correlation (CAZAC) sequence. The CAZAC sequence has superior power-derating characteristics, and can easily make an orthogonal sequence set using a circular shift (CS).
In this case, a correlation value between the CAZAC sequences to which different circular shifts (CSs) are applied is set to “0”. The orthogonal sequence set is indicative of the set of sequences, each of which has the corresponding value of “0”.
In association with the above-mentioned description, the degree of CS available in the same CAZAC sequence is defined by a zero-correlation zone (ZCZ). The ZCZ width is determined within a predetermined range in which a reception end has no difficulty in distinguishing the CAZAC sequences.
Besides the above-mentioned advantages, the CAZAC sequences have a very low cross-correlation value between random sequences, so that they can be distinguished from each other.
The 3GPP LTE has defined that the above-mentioned CAZAC sequences can be applied to the RACH, and has assumed that the CAZAC sequences can be repeatedly extended according to the cell sizes.
In other words, a given basic sequence is firstly designed to be suitable for a given RACH length. If a longer spreading gain is required, the basic sequence may be repeatedly used.
However, if the sequence is repeatedly used, the 3GPP LTE does not define how to extend each basic sequence. Therefore, if the sequence is extended in the form of an iterative sequence, the 3GPP LTE has difficulty in determining whether or not to repeat the CP along with the preamble, and also has difficulty in determining how to set the number of iterations. In addition, the 3GPP LTE has difficulty in determining the length of CP or ZCZ contained in the sequence, and has no solution of how to decide a signal transmission method.